Jacob:
Interesting concept. I have a question however.

Does your connection at the A/D converter get messed up when the RS485 bus goes to its extreme of common mode voltage range. If not please let me know how you handled this issue. I like your idea so much I want to try it.....but I anticipate a problem just hooking the two RS485 lines directly to a 0->5V A/D input when the RS485 bus may be experiencing -7 volts of common mode voltage relative to the the ground.

Mike